The Odds on Centauri

by Paul Gilster on December 30, 2008

My friend Tibor Pacher has taken our interstellar bet to a new level, publishing a lengthy letter on the subject in the current Spaceflight, a journal published by the British Interplanetary Society. Tibor, remember, had made a prediction I found outlandish: That “the first true interstellar mission, targeted at the closest star to the Sun or even farther, will be launched before or on 6 December 2025, and will be widely supported by the public.” I dissented, and we went public with the bet on the Long Bets site. Our funds are in the hands of the Long Now Foundation, with all proceeds going to good causes (details on the site).

But while I have enjoyed tweaking Tibor about the bet, it must be said that he has a solid motivation for going so far out on the speculative limb. The visionary founder of peregrinus interstellar, Tibor hopes to provoke discussion and keep people thinking. Along those lines, then, let’s look at his recent letter. One of the mission specs was a flight time of 2000 years or less to the star of choice. Assuming this is Proxima Centauri simply because of its, well, proximity, we arrive at a minimum average mission velocity of about 650 kilometers per second. That can be compared to Voyager 1’s 17.1 km/s to get an idea of the upgrade in velocity needed, but as we’ve noted in these pages before, the right kind of sail employing a Sun-diver maneuver might get at least close to that speed.

Useful data along the way? Tibor names the targets of opportunity: A craft traveling at 650 km/s gets out to the Kuiper Belt in about a year and reaches the heliosheath at 100 AU. Year two takes it out of the heliosphere entirely, while years five to ten are of note because they take us to the distance of the Sun’s gravitational focus, where Sol acts as a unique lens to magnify distant starlight. Recall that unlike optical lenses (where the light diverges after the focus), a gravitational lens has a focal line that extends to infinity. In other words, separations greater than 550 AU (where the gravitational lensing effect is first available) still offer unique observational possibilities.

Greg Matloff writes about this in his Deep Space Probes book (Springer/Praxis, 2000), noting that beyond 550 AU, the electromagnetic radiation from the occulted object under study is amplified by a factor of 108. And note this:

The ‘spot radius’ (distance from the centre line of the image at which the image intensity gain falls by a factor of 4) has been calculated…to be about 11 km for a Sun-spacecraft separation of 2,200 AU.

In other words, an outbound probe making lensing studies has a long observational run ahead of it. On the other hand, on a Proxima Centauri trajectory, what exactly will it be looking at? This is a rhetorical question, as I don’t know what is exactly on the opposite side of the Sun from this trajectory. Maybe one of our resident astronomers can fill me in.

Somewhere around year 20 of the Pacher probe’s mission it reaches the Oort Cloud, an area of obvious interest that may, in fact, extend halfway to the target star. We might also mention the Pioneer anomaly, for an outbound Proxima Centauri probe can obviously be studied in terms of anomalous acceleration along its route. Two thousand years after launch, the probe reaches the Proxima Centauri system, but for those who object that surely faster probes would have passed it along the way, I can only agree with Tibor that such a probe would get much done along its route before that happens, given a properly configured mission.

No, I won’t object to a 2000-year Centauri mission, powered perhaps by sail technology, on the grounds that it would quickly become obsolete. We don’t know what the future holds, and pushing the state of the art will teach us many things we would not otherwise have learned. Tibor and I have grounds for a bet, though, on the audacious idea that this mission might fly by December of 2025. If that occurs, I suspect advances in nanotechnology will be a large part of the story, but I will still be amazed if Tibor is the one to pop the Champagne cork when we meet seventeen years from now in Budapest to seal the deal.

I would love to see the 2,000 year 650 kilometer/second mission launched. This would be mission seems like one of the more doable missions in the near term. The math for the suggested mission is good.

I could imagine a world ship or space ark type of craft being powered in such a manner. 650 kilometers, I believe, is greater than the escape velocity of the Milky Way, and so for really ambitious extragalactic travel schemes and extragalactic colonizing endeavors, such a ship might be doable.

Such a world ship would be huge and would entail a huge R&D effort, however, the small velocity of the ship might result in greatly reduced risk of mission failure due to impacts with interstellar debris.

A humanity determined to survive and leave its influence or mark on the Cosmos forever night well be tempted to build such huge world ships and launch them in the comming centuries and millenia.

I certainly hope that warp drive and FTL travel becomes possible, but even 650 km/sec would or should enable mankind to venture to other star systems.

Given the long flight time and the likelyhood of this mission’s probe being overtaken, a better bet would be to launch in the exact opposite direction of the best Earthlike planetary candidate available so we could get images of an extrasolar Earth.

If a 10 to the 8 magnification is the equivalent of moving the object that distance closer, then a planet 100 ly away would be bought to the distance of 10 to the 7 km, the sort of distance even modest-sized optices could map the body with.

If we had a solar sail powered craft that left the solar system at 325 km/sec, it would reach observation distance in 10 years. If we could produce these missions at reasonable cost, then every time our next generation of telescopes picked up a couple of pixels of Earthlike planet with an interesting spectra, we could send a probe out to examine it.

Imagining extrasolar planets would then be done on a regular basis with the sort of timeframe and cost of missions to the outer solar system today.
Dave.

I am now skeptical about purpose of the mission. Technologies that will be able to provide spectroscopic data (and even low-res maps!) on Centauri planets are much closer to us then possible propulsion required for this mission. By 2025 we will surely have space telescopes good enough to determine all important information on the possible planets there, including potential for life and colonization.

Unless humans achieve truly outstanding propulsion breakthrough (like efficient FTL travel) that would be simply no need to send the probe. We can get all sufficient information about the exoplanets from Earth orbit, without waiting 2000 (or even 20) years. If we ever send interstellar probes, I assume they will have at least 0.1c speed to get there in reasonable time.

Given the coordinates of Proxima Centauri (Right Ascension: 14h 29m 42.9487″, Declination -62° 40′ 46.141″ – in J2000 Coordinates) it’s possible to figure out the “antipodes” of it on the sky as seen from Earth – or, to an approximation, from anywhere in the Solar System. It would be at J2000 coordinates R.A. 2h 29.715′, Dec. +62° 40’46.141″. This is a region in eastern Cassiopeia, about 4° E of the 3rd-magnitude star Epsilon Cassiopeiae. The star is a blue supergiant. Also nearby, though, is the type F main-sequence star HIP13665. Too, nearby are the open cluster IC 1805 and the emission nebula IC1795. And, of course, numerous stars beyond …

Would nice to see that one fly. I imagine an ultralight CNT star-sail would be the most likely to fly in the timeframe allowed, but like you I’m very sceptical. In 2000 years Alpha Centauri will be a bit closer to us so the transit speed has to be a minimum of 625 km/s, not 650.

…It would be at J2000 coordinates R.A. 2h 29.715′, Dec. +62° 40′46.141″. This is a region in eastern Cassiopeia, about 4° E of the 3rd-magnitude star Epsilon Cassiopeiae. The star is a blue supergiant. Also nearby, though, is the type F main-sequence star HIP13665. Too, nearby are the open cluster IC 1805 and the emission nebula IC1795. And, of course, numerous stars beyond …”

December 30th, 2008 at 16:47
I am now skeptical about purpose of the mission. Technologies that will be able to provide spectroscopic data (and even low-res maps!) on Centauri planets are much closer to us then possible propulsion required for this mission. By 2025 we will surely have space telescopes good enough to determine all important information on the possible planets there, including potential for life and colonization.

Unless humans achieve truly outstanding propulsion breakthrough (like efficient FTL travel) that would be simply no need to send the probe. We can get all sufficient information about the exoplanets from Earth orbit, without waiting 2000 (or even 20) years. If we ever send interstellar probes, I assume they will have at least 0.1c speed to get there in reasonable time.
[/quote]

I think a world ship/space ark type vessel could evolve quite naturally out of a technological trajectory that begins with simple orbital habitats. Building orbital space colonies would probably be no more challenging, perhaps less so, than establishing a moon or mars colony. The first colonies would probably stay in earth orbit and not be entirely self-sufficient, requiring regular injection of resources and people from earth. But over time, economic and political forces would likely compel at least some of these colonies to make themselves increasingly less dependent on earth, to the point of complete self-sufficiency, where they’d be capable of navigating around the solar system, mining resources from asteroids and comets. Eventually, some of them would develop the ability to “self-replicate,” essentially building another colony into which its excess population could move, after which the two colonies part ways.

Over time, a cloud of these colonies would gradually expand outwards from the inner solar system, all the way to the Oort Cloud, and then into the comet clouds of adjacent stars, and from there, some would inevitably choose to migrate down the gravity well into the inner systems of those stars.

Humanity could populate interstellar space in this fashion without ever having to summon the resources or will for some giant interstellar exploration project. It would simply happen naturally, as a byproduct of gradual population expansion, in the same manner in which our ancestors first left Africa and peopled the continents of earth.

What about Star Wisp, designed by Robert Forward. The materials for it are either here, or soon to be here with mass production of nano based carbon materials. I would much rather spend the money, assuming it’s available on advancing promising fast trip technologies, than wasting it on a trip that will sooon be passed by other technologies that will accelerate past it (both figuratively and literally)

What about Star Wisp, designed by Robert Forward. The materials for it are either here, or soon to be here with mass production of nano based carbon materials.

Great question. I loved the Starwisp idea, too, but Geoffrey Landis has convincingly shown that the design as Forward originally envisioned it would not stand up to the microwave beam — the spider-web structure of the probe would be incinerated! However, microwave beaming has a great future — as Jim Benford can attest, and more on my recent talk with him soon — and as materials evolve, things may look better. Carbon nano-materials are clearly a strong possibility, so that we may yet get some kind of descendant of Starwisp.

I join the others who think that the argument cannot be made for the need to get to the gravitational lens point within a 5-10 year time frame. I think that a 30-50 year time frame would be more acceptable if it makes a significant difference in the cost of the mission. A 2,000 year science probe mission to Proxima Centauri makes no sense due to the Wait Equation. A 2,000 year world ship is well beyond our current capabilities, will, and has an unacceptable risk of failure with the death of the travelers if we were using near-term technology.

I only see a 2,000 year mission being justified if it were to contain non-living humans in the form of frozen cells launched for the purpose of establishing a new civilization as an insurance against the self-extinction of the human species.

Thanks for depressing me with the oort cloud reference. After reading articles on the oort cloud, I realized how dangerous an interstellar flight is. With upward of 100 earth masses in a loose orbit up to 1 light year. This makes me realize how much has been lost to interstellar space. And think of all the material that has been created around other stars. Interstellar space must be litered with small object. This will pose a problem with very large ships moving a great speeds. So once again thanks for depressing me.

When the British Interplanetary Society’s starship team created the Project Daedalus design back in the 1970s, a key factor was damage control. The planners realized that, at 12 percent of the speed of light, their spacecraft could be destroyed by impact with the tiniest object. Dust itself could vaporize key structural materials. A more pervasive danger would be bremsstrahlung radiation, created when charged particles are suddenly slowed or deflected, forcing designers to shield sensitive electronics.

The is no mention of optical telescopes as the scope has to be pointed at the sun; however, if something like the starshade chronograph was used to block out the sun, then an optical/IR scope could be used.

The results of gravitational lensing are so mind-boggling as to be hard to believe. Maccone’s essay has tables showing image size a resolution for various wavelengths and a twelve meter dish. With short wavelength radio waves, a resolution of 81 km is possible at Alpha Centauri. If we used a similar sized dish at optical wave lengths, then a theoretical improvement of 10^4 is possible, meaning its now down to 8 meters (Large boulder size). While a 12 meter optical telescope at 1000 au with associated star shade is a massive undertaking, it is nothing like building a 0.1c interstellar ship.

The results of such a 12 meter telescope are mind boggling. It would have sufficient resolution to pick up planets around stars in the Andromeda galaxy. The device reminds me of Piers Anthony’s Macroscope, a device where you can see anything anywhere in the universe (just look a Maccone’s table for the resolution at 10 billion light years.)

If you find the results of gravitational lensing hard to believe, there have already been some spectacular results lensing distant stars with intervening faint red dwarfs. Double stars provide particularly good magnification, and they have got a transverse spectra of a red giant some 25,000 ly away. Only one other star has had its transverse spectra taken showing a cross section of the star and limb darkening: the Sun.

What I really need to do, Benjamin, is to get Claudio Maccone to answer these questions. I’ll be seeing him this summer at the Aosta conference, but let me see if I can get some answers before then. Re question 3, keep in line that the focal line extends to infinity — how this translates in practical terms to particular spacecraft options is going to be quite interesting to work out!

about student contests commenting the bet and I am referring to as well in my Spaceflight letter, I believe we have a good time to write down the various scenarios discussed here – many thanks to Paul for having this opportunity at all! – in a short but consistent way, going one step further and paving the way for more detailed analyses. The PI Club offers a venue for such work in the “Crazy Ideas” section – You might take a look at this here. I note that the first “Crazy Presentation” will be published soon, in the Category “Mission design”.

Does quantum entanglement have a future in interstellar communications??? I have heard that once entangled, there is the possibility for instantaneous communications no matter the distance. Actual implementation is another matter to be worked out but I think we are making amazing progress toward this. Thoughts??

Dave Moore Says: December 31st, 2008 at 17:02
“The results of such a 12 meter telescope are mind boggling. It would have sufficient resolution to pick up planets around stars in the Andromeda galaxy”

Yes, fascinating indeed: if magnification is 10^8, then the Andromeda galaxy would seem only about 0.03 ly away. Easy for a good optical/IR telescope or interferometer then to image and spectro-analyze earthlike planets in Andromeda.
And I presume that the distance of Andromeda would be an advantage here: a relatively small change in position of the craft would bring a lot more stars into view.

The Daedalus interstellar probe would also be equipped with a
“dust bug” that would spray fine material ahead of the main ship.

Objects up to one ton would be vaporized by the impact before
they hit the probe, with the beryllium shield presumably taking
care of any leftover debris.

And Daedalus would also have independent robot caretakers called
Wardens which will repair the vessel as required.

Just as we no longer consider Barnard’s Star as a first candidate
for an interstellar mission, if we find a closer brown dwarf (and
some of those objects have their own planets), will we go after
that before Alpha Centauri? That system does not seem to be
terribly promising for planets these days.

Perhaps that is our other error, focusing on Sunlike stars with
planets. If something is truly intelligent and has interstellar
capabilities, it may not be hanging around a planet.

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last seven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

On Comments

Centauri Dreams publishes selected comments on the articles under discussion here. Among the criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. Centauri Dreams is emphatically not a soapbox for political or religious views submitted by individuals or organizations.